1. Field of the Invention
The present invention relates to a zoom lens for suitable use in a camera (image taking apparatus), such as a broadcast television camera or a video camera, and to an image taking system including the zoom lens.
2. Description of the Related Art
In recent years, broadcast television cameras have shifted to HDTV (high definition television) cameras, and image taking systems that can realize higher-definition images are demanded. There is also a demand to reduce the total size and weight of the image taking system.
In order to meet these demands, the definition of image pickup elements has been increased, and the performance (resolution) of zoom lenses used in image taking optical systems has been increased. This makes it possible to take images having high frequency components.
However, since the focal depth of the image taking optical system decreases with the increase in resolution, fine focus adjustment is required near the in-focus position (or best image plane).
In the case of manual focusing, a user of the camera performs focusing while viewing a relatively small screen such as a view finder. In this method, it is difficult to accurately adjust the focus.
For this reason, there are strong demands for image taking optical systems having a high-accuracy autofocus function (auto focus detecting function).
Autofocus methods are roughly classified into active autofocus methods and passive autofocus methods.
In an active autofocus method, a ranging system (measuring system) is provided separately from an image taking system. Distance information about an object is obtained, for example, by emitting infrared light from the ranging system toward the object and receiving the infrared light reflected by the object.
The active autofocus method is not suitable for use in an image taking system of a television camera, since the system takes an image of a subject provided at a long distance and is required to have high mobility.
In contrast, Japanese Patent Laid-Open No. 2002-365528 discloses a passive autofocus method in which a part of a lens unit in an image taking optical system or an image pickup element is driven (wobbled) along the optical axis by a small amplitude so as to detect the contrast (sharpness) of a taken image for focusing. This method can obtain a signal for determining the direction of the best focus position (in-focus position), and is called a contrast method (so-called hill-climbing autofocus method).
In the method disclosed in Japanese Laid-Open No. 2002-365528, a prism for taking out a part of image taking light is provided near an eyepiece window of the finder so that auto focusing can be performed over the entire image-taking angle of view.
In another passive autofocus method, a part of a light beam traveling in the optical path of a picture taking optical system is split by a splitting element (beam splitting element), and the split part is focused onto an image pickup element for focus detection so as to obtain a focus signal.
Methods of thus obtaining a focus signal by using a splitting optical system (focusing optical system) different from the image taking optical system are widely known, as disclosed in U.S. Pat. Nos. 7,209,175 and 7,345,706 and U.S. Patent Application Publication No. 2003/0174232.
In these methods, in which a splitting optical system is provided separately from the image taking optical system, it is possible to obtain a focus signal without deteriorating the quality of taken images.
When the splitting element for splitting a part of a light beam is provided in the optical path of the image taking optical system and the direction of the in-focus position is determined using the splitting optical system separate from the image taking optical system, operation of seeking the in-focus position is not reflected in the taken image.
However, the addition of the splitting optical system in the optical path of the image taking optical system increases the total length of the lens. In addition, this also increases the weight of the lens, and reduces mobility of the image taking system. Further, the effective diameter of the lens increases, and the position of the exit pupil becomes close to the imaging position. For example, when the image taking system is applied to a 3-CCD television camera having a color separation optical system, white shading occurs, and it is difficult to obtain high optical performance.
For this reason, when the splitting element is provided in the image taking optical path, it is important to minimize the optical path length of the splitting element.
In some zoom lenses for television cameras, a lens unit in a relay optical system (imaging unit) is partly inserted and removed into and from the optical path in exchange for another lens unit (extender lens), thereby changing the focal length range of the entire lens system.
In this case, the f-number of the entire lens system is changed by insertion or removal of the extender lens, and the focal depth also changes therewith. When the focal depth changes, it is difficult to obtain a high focus detection accuracy.